Why did USA find fighting with Vietnam difficult?

This is for sure very persistent question because it was the first lose in USA history. So why does it happened?

It might be several reason why finally USA lose in Vietnam War.

First and in my opinion one of the most important is well known fact that it is always easier to defend your country because of strong motivation than to attack foreign nation. In this case motivation of Vietnamese army was much stronger than the USA soldiers mostly because of the fact that in case of losing enemy will control their country.

Another thing was that USA soldiers were foreigner. Slightly to less of them knew anything about Vietnam and non of them were able to speak Vietnamese. They were fighting in un-known terrain and most of them were not trained for defending against guerilla warfare. The case was also about the climate that American army weren't prepared for. Such heat and humidity, new conditions, make them fight worst.

But they didn't really care about preparation before coming because they thought to stay in Vietnam just for a while. Apart from expectation war last longer which make american people tired and wanted quickly back home what have a big influence for level of involvement. They didn't really have strong target they just stayed there of coercion. So with the time they started to take drugs which were very common and easily available they stopped care about war which has crippling effects.

They lose also because of Guerilla war that they didn't expected. American troops were mostly supposed to fight NVA (North Vietnamese Army) but finally they had to stay and defend against guerilla warfare which was much harder because enemy knew terrain better while for them was absolutely new. The main problem in this case was based on fact that USA army didn't exactly know who shot to. Vietnamese soldiers didn't vary from civil so it was hardly impossible to check who were really who. Because of that lots of innocent people died and war from people point of view starts to be inappropriate and cruel which soon cause in American government to be very unpopular.

At least USA didn't even have a lot support from allies as South Korea, Thailand, Australia, New Zealand and Philippines. The amount of soldiers that was sent for help was ridiculously small. The number of american who were killed during the war was ten times bigger than all allies soldiers which in total has also a contribution in course of war.

Causes of Vietnam war

1. American anti-communism - the involvement of America into Vietnam started because it was afraid of communism. As soon as the war broke out between south and north they started to be afraid of Vietnam becoming a communist country because this political system would start to dominate in most Asian countries. (according to domino theory). The domino theory suggests that if one country of the south-east Asia will be under the control of the communists, in consequences this system will spread out to all countries around, just like in a domino game. (if one piece fall, it start the whole series of 'falling')
2. War between North and South - to this conflict were involved South (Republic of Vietnam) and North (Democratic Republic of Vietnam). South which seemed to be against communism were supported by USA and their allies South Korea, Thailand, Australia, New Zealand and Philippines and North were supported by communist countries as Soviet Union. As soon as South Vietnam started to lose America gave them big financial support which very quickly proved to be not enough. So they gave them military advisors but  as it didn't help they then started to send American soldiers. 
3. French return in 1945 and Vietnamese resistance - 2 september 1945 the Democratic Republic of Vietnam made an uprise??. Vietnam was then a French colony but France didn't even think about resign from the Indochina. As a result they fight bloody for several years. In the final stage of this war France was supported by USA even with american airman and military advisors. At the end Vietnam defeated France so they sign a peace treaty with communists from Viet Minh. After this Vietnam had been devided into two parts.